Described herein are documents and methods of making and using the same, wherein the document contains a substantially invisible image thereon. The presence of the hidden information is very difficult to detect by persons not knowing that the hidden information is included in the document. Thus, the hidden information is able to remain secure except to persons aware of its presence, and the document can be very difficult to counterfeit, thus making the document advantageously useful in security applications.
An especially difficult task with document security is the creation of documents that contain embedded or hidden information the presence of which is not detectable by the naked human eye. One technique being used is to utilize covert digital encryption in which the information is encoded and printed in encoded format on the document. However, because of the encryption, this technique requires the users to also utilize specially dedicated electronic reader translating software.
Another technique being utilized is to print the information onto the document with clear (colorless) inks that include materials that interact with UV light, for example by fluorescing, so that the information can become visible to the naked human eye upon exposure to the UV light.
For example, U.S. Patent Publication No. 2004/0233465 describes an article marked with image indicia for authentication, information, or decoration by providing a plurality of inks having a plurality of fluorescence colors when exposed to excitation energy, separating colors of the image indicia into a plurality of image levels in accordance with the fluorescence colors of the inks, and printing each image level in mutual registration on the article using the corresponding ink. The image printed with each ink may be substantially invisible under illumination within the visible spectrum. The invisibly printed images have multiple authentication features, including the use of covert UV-fluorescent materials, IR-fluorophores, microparticles, and other chemical taggants.
U.S. Pat. No. 5,807,625 describes photochromic printing inks that are used for the printing of security documents. Prints are normally nearly colorless and become colored when energy irradiated, such as by ultraviolet light. This photocoloration is reversible. The printing inks contain photochromic compounds which are protected against other ink components. Methods are described to prepare the inks, to print security documents, and to detect counterfeiting.
However, several problems may be encountered with the above technique. First, the clear ink used to form the hidden information may have a differential gloss from the document substrate, typically paper, and thus the naked human eye could detect that something is present on the document. A counterfeiter could then investigate further to reveal the hidden information. Second, even if no differential gloss were evident, the hidden information may still be revealed with the use of a simple black light, and counterfeiters knowing of the prevalent use of UV absorbing inks often will check a document under black light.
What is still required is a simple method of embedding hidden information or images into a document such that the information is substantially undetectable to the naked human eye due to differential gloss, and which is further not detectable or revealed by black light.